The present invention generally relates to a state of charge indicator for providing an indication when the state of charge of a depletable energy source falls below a predetermined threshold. The present invention is more particularly directed to a state of charge indicator for monitoring a voltage produced by a battery used in a deep cycle application and producing an indication when the state of charge of the battery falls below a predetermined threshold.
The state of charge of a depletable energy source such as a battery is an indication of the relative amount of useable energy remaining in the battery. The state of charge is given as a percentage equal to the ratio of the present amount of useable energy remaining in the battery to the amount of useable energy in the battery when it is fully charged. Thus, a fully-charged battery has a 100% state of charge, a fully-discharged battery has a 0% state of charge, and a battery charged to a level equal to one-half its full state of charge has a 50% state of charge.
Batteries are often used in deep cycle applications to provide electrical energy to power electrical devices such as electric motors or emergency lighting. In deep cycle applications, batteries are routinely and repetitively discharged to a relatively low state of charge and recharged to a near-100% state of charge. In other applications, batteries such as starting batteries in automobiles, for example, are used only intermittently, to supply electrical energy to the starter motor of the automobile. Aside from these momentary intermittent usages, such batteries are generally maintained near 100% state of charge. Deep cycle applications, in contrast, routinely discharge the battery to a low state of charge during normal usage.
Typical deep cycle applications for a battery include providing power to an electric trolling motor in a fishing boat, providing power to the electric drive system of a battery-operated wheelchair, or emergency power for security systems. In such applications, the battery may be required to supply current for several hours. Supplying this current will discharge the battery to a relatively low state of charge. Later, the battery may be recharged to a nearly 100% state of charge. This same cycle of discharging and recharging the battery may be repeated many times over the life of the battery.
In view of these typical applications, it would be advantageous to provide a battery with a state of charge indicator. Such an indicator would provide a visible or audible indication when the state of charge of the battery has fallen below a predetermined threshold. The indication would inform a user of the low state of charge condition and the impending need to recharge the battery. The indication reduces the risk of discharging the battery to a level insufficient to provide usable power or to a level at which irreversible damage to the battery may occur.
Prior art state of charge indicators are not well-adapted to deep cycle battery applications. U.S. Pat. No. 5,130,699, issued to Reher, et al., on Jul. 14, 1992, and assigned to the assignee of the present invention, discloses a digital battery capacity warning device. The device periodically compares the output voltage produced by a battery with a predetermined threshold. The result of each comparison is stored. If three or more of the last 16 comparisons indicate a low voltage, an indication of low state of charge is provided. An indication is provided approximately 1.5 hours after the low state of charge condition occurred.
Such prior art state of charge indicators may not respond rapidly enough to the decreasing state of charge of a battery used in deep cycle applications. Because of the applications for deep cycle batteries noted above, including marine, wheelchair, and security applications, timely indication of a low state of charge condition is preferable. A 1.5 hour delay in providing a low state of charge indication could be intolerable in such applications.
Moreover, such prior art state of charge indicators are adapted to monitoring the state of charge when the battery is at rest and not subjected to a load current. For batteries utilized in deep cycle applications under normal operating conditions, the battery does not typically remain at rest for substantial periods. Rather, the battery may supply currents across a large range, such as 0 to 100 amperes. Accordingly, there is a need for a state of charge indicator for deep cycle applications which can provide a timely indication of a low state of charge condition when the battery is subjected to a wide range of load currents.
There is also a need for a low state of charge indicator which is not sensitive to voltage fluctuations which might cause the state of charge indication to fluctuate about the low state of charge set point. The state of charge indicator may provide, for example, an indication when the battery state of charge falls below a set point of 50% state of charge. As the state of charge of the battery falls below this set point, a transient in the current supplied by the battery may cause the voltage supplied by the battery to falsely indicate a state of charge above the set point. Accordingly, there is a need for a state of charge indicator which latches the low state of charge indication and does not reset the latched condition until the battery has been recharged to well above the state of charge setpoint level.
The state of charge indicator of the present invention provides solutions to these noted deficiencies in prior art state of charge indicators. In general, the state of charge indicator of the present invention measures battery voltage and compares the battery voltage to different preset voltage values at corresponding specified time intervals and evaluates if the battery voltage is above or below the preset voltage values. The preset voltage values and corresponding specified time intervals are empirically determined for a given type of battery and state of charge setpoint level. The results of the evaluations are placed into, for example, shift registers. The number of low indications within the shift registers are summed and evaluated. If the sum exceeds a predetermined accumulated status threshold, an output indication is provided, indicating a low state of charge condition. If the low state of charge indication is provided, this output indication is latched until the state of charge indicator detects that the battery has been recharged to well above the predetermined state of charge setpoint level.